Most commercial metal detectors are designed for hunting buried metal targets such as gold, coins, treasure and archaeological artifacts. Most of these have transmit electronics arranged to generate a repeating transmit signal cycle, whose output is connected to a transmit coil for the transmission of alternating magnetic fields. Most include a magnetic field receiving means such as a receive coil which is connected to receive electronics which includes signal processing and assessment electronics to give an indicator output. The signal processing and assessment electronics usually includes a preamplifier which output is connected to synchronous demodulators which synchronous demodulation multiplication functions are synchronised to the transmit signal. The outputs of the synchronous demodulators are connected to low-pass filters or “demodulation filters”. The low-pass filter outputs are further processed for target identification and indication. The demodulation, low-pass filtering, and further processing may be implemented in analogue and/or digital (DSP).
The principal mode of operation of coin and treasure detectors is “discrimination” mode where ferrous targets are usually discriminated against, but non-ferrous targets within certain time constant ranges are not discriminated against and indicated usually by an audio signal with an accompanying visually displayed time constant. In order to improve the time constant assessment, some metal detectors simultaneously transmit and receive several frequencies, most in the form of multi-period pulse transmission. However, pulse induction (e.g. U.S. Pat. No. 5,576,624) and multi-frequency sine-waves (e.g. U.S. Pat. No. 4,942,360) will suffice too.
Some commercially available sinusoidal single-frequency transmitting metal detectors have switches which allow a user to select different frequencies. The electronics in such detectors is often relatively expensive. The ability to select different frequencies may be useful, for example, in gold nugget prospecting where the size of gold nuggets may vary from location to location and hence the optimal frequency for detection may also correspondingly vary.
Detectors that operate in the “time-domain” contain switching electronics which switches various voltages from various power sources to the transmit coil for various periods. U.S. Pat. No. 5,537,041 discloses a metal detector which transmits multi-period pulses and operates in the time-domain, as too do some commercially available pulse induction metal detectors. Also see for example U.S. Pat. No. 5,576,624.